The United States and its regional allies are fully prepared to counter any threat from North Korea, US Secretary of State John Kerry and the Pentagon say.
"We are prepared to take whatever measures are necessary to ensure our alliances continue to defend against this growing threat to international peace and security," Kerry said in a statement on Friday.
The statement came after North Korea announced earlier in the day that it had conducted a successful "nuclear warhead explosion" test, saying it was meant to counter US hostility.
“The 10-kiloton blast was nearly twice the fourth nuclear test and slightly less than the Hiroshima bombing, which was measured about 15 kilotons,” said Kim Nam-Wook of South Korea’s meteorological agency.
Pentagon spokesman Captain Jeff Davis also said Friday that “this is a threat we recognize as very real and we are always ensuring that we are outpacing it.”
"We take them at their word and have built a defensive architecture that assumes that they have the capability -- but we have never actually seen it demonstrated."
The US is working with South Korea to install a missile system known as THAAD in the country.
Davis said the THAAD system would be ready in 2017, but added that it could be installed quicker in case of emergency.
"There is a lengthy process we are working through with (Seoul) but of course... we can always move it much more quickly," he said.
President Barack Obama also said Friday that he would push for new sanctions in retaliation to Pyongyang’s fifth and most powerful nuclear test.
The UN and the West have so far imposed a raft of sanctions on North Korea, prompting Pyongyang to step up its nuclear activities.
On Monday, North Korean leader Kim Jong-un called on the country's military to bolster its nuclear capabilities after ordering the launch of three ballistic missiles.
In January, North Korea said it had successfully detonated a hydrogen bomb, its fourth nuclear test, and vowed to build up its nuclear program as deterrence against potential aggression from the US and its regional allies.
North Korea says it will not give up on its nuclear “deterrence” unless Washington ends its hostile policy toward Pyongyang and dissolves the US-led command in South Korea. Thousands of US soldiers are stationed in South Korea and Japan.